Fuel cells, and in particular metal-air battery systems, have long been considered a desirable power source in view of their inherent high energy density. A fuel-cell battery includes a cathode, an ionic medium and an anode. A metal-air cell employs an anode comprised of metal particles that is fed into the cell and oxidized as the cell discharges. The cathode is generally comprised of a semipermeable membrane, a mesh of inert conductor, and a catalyzed layer for reducing oxygen that diffuses through the membrane from outside the cell. Since oxygen is readily available in the air, it is usually unnecessary to utilize a dedicated oxygen storage vessel for the fuel-cell battery (except in certain configurations where there the oxygen supply is limited due to design considerations). This makes metal-air cells very efficient on both a volumetric energy density and cost basis. The cathode and anode are separated by an insulative medium that is permeable to the electrolyte. A zinc-air refuelable battery consumes zinc particles and oxygen as zinc is oxidized by the reaction with ions passing through the electrolyte while liberating electrons to produce electricity. The reaction products are generally comprised of dissolved zincate and particles of zinc oxide suspended in the spent electrolyte.
Prior art metal-air systems have been demonstrated with sufficient energy capacity to power electric vehicles. Such metal-air batteries having recirculating metal slurry anodes were built for demonstration purposes in the 1970s by Sony, Sanyo, the Bulgarian Academy of Sciences, and the Compagnie General d'Electricitie. These systems never achieved any commercial success because they all had relatively low power output (acceptable drain rates and overall capacities). Until now, this has been the major obstacle to providing a commercially viable system. For example, Sony could only provide 24 W/kg, and Compagnie General d'Electricitie was limited to 82 W/kg or 84 Wh/kg. The theoretical capacity, however, is well in excess of five times these values depending upon the type of fuel utilized. One type of recent metal-air cell has realized an improvement in capacity by utilizing a packed bed of stationary anode particles and an electrolyte which moves through the bed without the use of an external electrolyte pump. Although this system has increased the cell capacity to about 200 W/kg with an energy density of about 150 Wh/kg, further improvements are necessary before commercial success will be realized.
Metal-air refuelable batteries can be refueled in a short amount of time (i.e., minutes), compared to the several hours typically required to recharge conventional batteries. This characteristic makes them very well suited to mobile applications such as electric vehicles, portable power sources and the like. During the refueling operation, fresh anode metal and electrolyte are added to the cell, and the reaction products and spent electrolyte are removed. The reaction products must be either transported to an industrial facility for recycling or used, as is, for another purpose. Several methods have been proposed for refueling metal-air cells. One known system employs two reservoirs, one to store fresh anode fuel and one to accommodate reaction materials from the cell.
U.S. Pat. No. 4,172,924 discloses a metal-air cell that utilizes a fluid metal fuel comprised of a mixture of metal particles and liquid electrolyte in a paste form. The paste moves from a first reservoir through the electrochemical battery where it is oxidized at a corresponding metal oxide paste cathode. The reaction products (primarily metal oxide) are communicated to a second reservoir. While this arrangement increases the drain rate by removing the reaction materials, the multiple reservoir design wastes space, adds complexity, and increases cost.
Recently issued U.S. Pat. No. 5,952,117 discloses a fuel cell battery designed to overcome the disadvantages associated with the dual reservoir configuration described above. The '117 patent discloses a transportable container for supplying anode material and electrolyte to the fuel cell battery, circulating electrolyte in a closed system, and collecting spent anode reaction product. In accordance with the teachings of this patent, the container is first filled with zinc fuel particles and fresh electrolyte. Next, the container is transported to the fuel cell battery and connected to the battery such that it becomes part of the electrolyte flow circuit. After the zinc fuel and electrolyte are used for a period of time during battery discharge, the container, now containing at least partially spent electrolyte and reaction products, is removed from the battery and transported back to the refilling apparatus. The contents of the container are subsequently emptied into the refilling apparatus and the process is repeated. The spent electrolyte and reaction products are regenerated at a zinc regeneration apparatus and then returned to the refilling apparatus. Although this arrangement obviates the need for two separate containers, the collection of reaction products can be made effectively only after the fuel supply has been exhausted and the container has been emptied into the refilling apparatus.
Another shortcoming of this system concerns the structure for preventing stray short circuit currents between a plurality of cells that are fed fuel in parallel. In that configuration, the cells are not electrically isolated from each other through the conductive fuel feed. To prevent short-circuiting, the '117 patent discloses a filter for blocking large particles of anode material from passing through the conduits between the fuel compartments. Although effective for the pellet-type fuel particles disclosed in the patent, this expedient cannot block the passage of the small anode particles that are found in a paste-like fuel substance.